When it comes to video games, Valve Software was one of the early game studios to emphasize the importance of interactive narrative in shooters like Half-Life. Storytelling remains an integral part of all of Valve`s games. And Erik Wolpaw is one of the top writers at the Seattle game studio.

Wolpaw began his career as a journalist writing for game sites like GameSpot.com and founding OldManMurray.com. That work led to a job at Double Fine Productions for Psychonauts, and in 2006 he was honored with a Game Developers Choice Award for best writing.

Gabe Newell hired Wolpaw at Valve, where he`s worked on the story and dialogue for games like Portal 1, Left 4 Dead and now Portal 2. Wolpaw talks about the creation of Valve`s much-anticipated Portal 2, which ships in April, in DIG`s exclusive interview.

DIG: For those out there who haven`t played the 2007 Portal, can you explain what this game is all about?

Erik Wolpaw:Portal 2 is a 3D action-adventure puzzle game in which you have a portal gun that shoots two interlinked, inter-dimensional gates that you can use to traverse space. You`re the test subject who`s the prisoner of an insane artificial intelligence, GLaDOS, that`s putting you through an increasingly dangerous and complex series of tests.

DIG: What were your goals heading into this sequel?

E.W.: After Portal 1 shipped, we noticed a lot of players were improvising a cooperative mode where two people would think through the puzzles with just one of them working the controller. We had co-workers who`d play the game with their wives or girlfriends or friends.

Going into Portal 2, we wanted to formalize that co-op style of play and get a controller into the second person`s hand. Portal 2 features a complete co-op experience with its own set of puzzles and its own story. For the single-player game, we wanted to expand the number of puzzle elements, introduce some new characters, and tell a surprising new story inside the fun house of science that is Aperture Laboratories.

DIG: What were the challenges of designing this co-op gameplay?

E.W.: We had to design co-op puzzles that one person couldn`t do on his own. It took the designers a little while to really wrap their heads around it. Environment tagging was the key to making co-op work, because it allowed players to visually communicate where to add a portal quickly. We had a lot of ideas that we couldn`t put into the original game and we were able to really explore a lot of those with this game in the co-op.

DIG: Can you talk about how improvements in technology and your team size enabled you to innovate with Portal 2?

E.W.:Portal 1 was made with seven or eight employees. With this game, we had over 30 people working on it. Besides the co-op play, one of the things we really wanted to do in the original game -- but didn`t have the manpower to do -- was to create a living, breathing facility. We always had this idea in our heads that the test chambers were modularly assembled and could be rebuilt.

Since players have destroyed Aperture Laboratories at the end of Portal 1, we used the premise of GLaDOS reassembling the facility to show players the destruction they`d caused. We also took advantage of the larger team by creating an environment that`s evolving in real time as the player is progressing through the puzzles.

DIG: What excites you about designing games for the PC gaming landscape today?

E.W.: Because it`s an open platform, the PC lets small teams try out some really innovative games. And because of digital distribution systems like Steam, these boutique developers can actually earn a living making really interesting niche titles. There`s never been a better time to be a professional game designer.

DIG: What impact did fan feedback have on the development of this game?

E.W.: The biggest piece of feedback we received from Portal 1 was that, while people really loved thinking through the puzzles, they generally didn`t enjoy it when the puzzles then required tricky execution skills to finish. They loved the "A-ha!" moment that occurs when the solution finally clicks, but were frustrated when they then had to struggle with the controls to put that solution into practice.

We addressed that complaint in two ways. First, we designed the puzzles so that they don`t require ninja reflexes to solve. Second, we tweaked the physics a bit so that movement through portals is a little more forgiving than in the first game.

DIG: How does this game compare to the original?

E.W.: It`s bigger along every axis. It features more puzzle elements, more characters, more environments, and a fully fleshed-out two-player cooperative mode. That said, our goal was to retain the core simplicity and elegance of the portal mechanic. We wanted the game to be more complex, but not harder. It`s still a long, gradual training arc that never presents players with a problem they`re not prepared to tackle.

Similarly, from a narrative perspective, we retained the smaller-scale, intimate nature of Portal 1`s story. At the end of the day, it`s still about the relationship between you and GLaDOS. Though this time, that relationship gets complicated in some surprising ways.

DIG: What will the experience be for a PC gamer who turns up the levers to full for Portal 2?

E.W.: It looks great. It`s a big visual upgrade from Portal 1, with GLaDOS reassembling Aperture Laboratories and creating an environment where things are always animated and evolving.

It`s a longer game, and a lot more epic. Adding new surface properties to the new game also adds something new to both the puzzle-solving gameplay and the look of the game. And the new paint system really works well.